Combined gasification and vitrification system

ABSTRACT

An optimized gasification/vitrification processing system having a gasification unit which converts organic materials to a hydrogen rich gas and ash in communication with a joule heated vitrification unit which converts the ash formed in the gasification unit into glass, and a plasma which converts elemental carbon and products of incomplete combustion formed in the gasification unit into a hydrogen rich gas.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/432,826, filed May 12, 2006, now U.S. Pat. No. 7,854,775.

TECHNICAL FIELD

The present invention relates generally to a method and apparatus forprocessing feedstocks containing organic materials. More specifically,the present invention relates to an integrated partial oxidationgasification and vitrification system which provides an improved methodand apparatus for recovering the energy value from such feedstocks whilerendering the inorganic portions in a safe and useable form.

BACKGROUND OF THE INVENTION

In the mid-1800s, biomass, principally woody biomass, supplied over 90%of U.S. energy and fuel needs. Thereafter, biomass energy usage began todecrease as fossil fuels became the preferred energy resources. Today,the world's energy markets rely heavily on fossil fuels, coal, petroleumcrude oil, and natural gas as sources of energy. Since millions of yearsare required to form fossil fuels in the earth, their reserves arefinite and subject to depletion as they are consumed. The only othernaturally-occurring, energy-containing carbon resource known that islarge enough to be used as a substitute for fossil fuels is biomass.Biomass is herein defined as all nonfossil organic materials that havean intrinsic chemical energy content. They include all water andland-based vegetation and trees, or virgin biomass, as well as all wastebiomass such as municipal solid waste (MSW), municipal biosolids(sewage) and animal wastes (manures), forestry and agriculturalresidues, and certain types of industrial wastes. It is understood thattoday's wastes consist of a mixture of materials derived from fossilfuels and non-fossil organic materials.

Unlike fossil fuels, biomass is renewable in the sense that only a shortperiod of time is needed to replace what is used as an energy resource.Some analysts now believe that the end of the Fossil Fuel Era is insight because depletion of reserves is expected to start before themiddle of the 21^(st) century, probably first with natural gas. Thiseventuality and the adverse impact of fossil fuel usage on theenvironment are expected to be the driving forces that stimulate thetransformation of biomass into one of the dominant energy resources.

Under ordinary circumstances, virgin biomass is harvested for feed,food, fiber, and materials of construction or is left in the growthareas where natural decomposition occurs. The decomposing biomass or thewaste products from the harvesting and processing of biomass, ifdisposed of on or in land, can in theory be partially recovered after along period of time as fossil fuels. Alternatively, virgin biomass, andany waste biomass that results from the processing or consumption ofvirgin biomass, can be transformed into energy, fuels, or chemicals. Thetechnologies for such conversion include a variety of thermal andthermochemical processes, gasification, liquefaction, and the microbiolconversion of biomass to gaseous and liquid fuels by fermentativemethods. Many of these processes are suitable for either directconversion of biomass or conversion of intermediates. The syntheticfuels produced by these methods are either identical to those obtainedfrom fossil feedstocks, or if not identical, at least suitable as fossilfuel substitutes.

One example of biomass conversion technology are techniques wherebybiomass is gasified by partial oxidation to yield a low-calorific-valuefuel gas, or synthesis gas, which may then be used as a feed stock inchemical synthesis processes, or as an energy source, for example, todrive an internal combustion engine, a gas turbine or a fuel cell togenerate electric power. Such schemes rely on exposing the organic feedstocks to heat and a limited amount of oxygen in specially configuredgasifiers to effect partial oxidation of the organic materials, therebyproducing an effluent gas consisting primarily of hydrogen and carbonmonoxide. Currently, hundreds of companies throughout the world offersuch systems for the production of such fuel gas. It is important tonote that such methods are effective in converting virtually all organicfeed stocks, including biomass, fossil-based organic materials, andtheir derivatives, including waste derived from the production and useof biomass and fossil-based organic materials, into electrical power.

In addition to the production of synthesis gas through partial oxidationin gassifiers, synthesis gas has also been produced using systems whichconvert water and organic materials into synthesis gas in a steamreforming reaction. Examples of some such systems include that describedin Production of Technological Gas for Synthesis of Ammonia and Methanolfrom Hydrocarbon Gases, Chemistry, A. G. Leibysh, Moscow, 1971. Thispaper describes the conversion of methane by steam without catalyst at avariety of different temperatures and different ratios of H₂O:CH₄. Thispaper, the entire contents of which are incorporated herein byreference, shows synthesis gas production in both pilot plantexperiments and lab results obtained from a quartz reactor. The generaltrend towards complete conversion of the organic feedstocks intosynthesis gas with increasing residence time and temperature is shown inboth a graphical presentation and in tables of the observed experimentaldata. Related experimental work in the United States was reported in“Synthesis Gas Production from Organic Wastes by Pyrolysis/SteamReforming” Energy from Biomass and Wastes:1978 Update, by Dr. Michael J.Antal, Jr., the entire contents of which are incorporated by reference.In this work, steam gasification of biomass is accomplished as a twostep process. At a relatively low temperature (300° to 500° C.) thebiomass is pyrolyzed, producing volatile matter and char. At somewhathigher temperatures (˜600° C.) the volatile matter is then reacted withsteam to produce a hydrocarbon rich synthesis gas. The Handbook ofThermodynamic Temperature Process Data, by A. L Suris, 1985, the entirecontents of which are incorporated herein by reference, shows thetheoretical products of the non-combustive decomposition of methane withwater (CH₄+2H₂O) across increasing temperatures. At 1000° C., thedestruction of methane is greater than 99%, and at 1400° C., thedestruction of methane is greater than 99.99%.

While these and other gasification systems have shown a wide variety ofbenefits, several drawbacks are still present in their operation. Forexample, these types of systems typically are not well suited toprocessing heterogeneous feed stocks, which are defined herein as feedstocks containing mixtures of organic and inorganic materials. In manycases, the inorganic constituents of the feed stocks can adverselyeffect the processing of the organic portion, resulting in less thancomplete conversion, or low processing rates. Also, the inorganicconstituents may be left in a highly concentrated ash form, renderingthem highly soluble into the environment, particularly ground water, andtherefore potentially environmentally hazardous and/or requiringexpensive treatment to stabilize these constituents prior to finaldisposal. Even when operated with homogeneous organic feedstocks,gasification systems typically have drawbacks. For example, a commontradeoff in the operation of a gasification system is between having aclean gas product and minimizing the residual organic product which mustbe discarded. Typically, a high quality gas is not formed if the organicfeedstock is completely gasified. Instead, various oils, tars and otherundesired components are present in the gas. Alternatively, a highquality gas may be formed, but only by having less than completegasification of the feedstock. This results in a waste product ofpartially oxidized organic material that must be disposed of, often atgreat cost.

A desire to destroy hazardous organic waste streams has led to their useas a feedstock for steam reforming systems. For example, U.S. Pat. No.4,874,587 to Terry R. Galloway, the entire contents of which areincorporated herein by reference, describes a system whereby organicliquids are first volatilized into a gaseous form. The volatilizedliquids are then mixed with an amount of water in excess ofstoichoimetery in the form of steam. This organic gas mixture is thenintroduced into a first reaction zone maintained at a temperaturebetween 200 and 1400° C. Within this first reaction zone, the steam andorganic gas mixture are directed through a “labryinthine path” whichpresents “organically adsorbent surfaces” to the gaseous mixture. Withinthis first reaction zone, the labryinthine path and adsorbent surfacesare “selected to provide sufficient temperature, turbulent mixing, andresidence time in the first reaction zone for substantially all of thegaseous organic compounds to react with the water.” “Substantially all”of the organic compounds is defined as in excess of 99% and preferablyin excess of 99.99% reacted. The gaseous mixture is then passed into asecond reaction zone having a temperature range higher than the firstand between about 750 and 1820° C. As was the case with the firstreaction zone, in the second reaction zone, the amount of water iscontrolled so that it is equal to or in excess of stoichiometry. Thespecification states that “the higher temperature of the second reactionzone, together with the lower level of organic compounds entering thesecond reaction zone, assure that total and complete reaction of theorganic compounds results to a level of at least 99.99% and typicallymuch higher.” [sic] The heating for the first and second reaction zonesis provided by a plurality of elongated U-shaped hairpin loops ofelectrical resistance heating elements located within the interior ofthe second reaction zone.

Systems such as that described by Galloway seek to provide a dualbenefit; the destruction of the hazardous organic materials and thecreation of a useful synthesis gas. Similarly, waste destruction systemssuch as that described by Galloway seek to address concerns related tothe production of so-called products of incomplete combustion, or PICs,such as hydrofurans and dioxins. To avoid the production of PICs, thesetypes of systems may be operated in reducing environments whereconditions for the production of PICs are not favored. In these systems,the energy required to drive the endothermic steam reforming reactionsmust be provided from a source external to the reaction. Since theenergy consumption of these external sources offsets the economicbenefit of the synthesis gas produced, the efficiency of delivering thisenergy is invariably an important consideration in the design of thesesystems. For this reason, the volatilization of the organic feedstockwith a first heating source, prior to steam reforming the resultant gaswith a second heating source, as described in both the Antal system andthe Galloway system, imposes a significant economic penalty on thesesystems.

Interest has been directed to the use of plasmas in these steamreforming systems. For example, a process similar to the Galloway systemis described in Hydrogen Production by the Hüls Plasma-ReformingProcess, G. Kaske, et al., Advanced Hydrogen Energy, Vol. 5, (1986), theentire contents of which are incorporated herein by this reference. Inthe Kaske system, a plasma is used to reform “gaseous hydrocarbons arereformed with gaseous oxidizing agents, such as steam or carbondioxide.” The systems described by Kaske thus suffer from many of thedrawbacks found in the Galloway systems, in particular, the limitationswhich arise due to the focus on steam reforming volatilized organicgasses, as opposed to solid or liquid feedstocks.

Plasmas are high temperature, ionized gasses which provide rapid andefficient heat transfer. The ability of plasmas to rapidly transfer heatto incoming organic feedstocks allows the plasma to simultaneouslypyrolize the organic feedstocks and provide the thermal energy to drivethe endothermic steam reforming reactions of the pyrolyzed organicfeedstocks. This dual benefit has been deployed with great success insystems utilizing plasmas including those described in U.S. Pat. No.5,666,891, titled “Arc Plasma-Melter Electro Conversion System for WasteTreatment and Resource Recovery” to Titus et al. and which the entirecontents are incorporated herein by reference, and which shows a varietyof particularly useful configurations wherein arc electrodes whichproduce the plasma are used in systems in various combinations withjoule electrodes. In these arrangements, organic compounds contained inthe waste are destroyed by pyrolysis, caused by the high temperatures ofthe plasma breaking the chemical bonds of the organic molecules. Byintroducing steam into the process chamber, these pyrolyzed organicconstituents are converted into synthesis gas, a clean burning fuelconsisting primarily of CO, CO₂ and H₂, through the steam reformingreaction. Other constituents of the waste, which are able to withstandthe high temperatures without becoming volatilized, are made to forminto a molten state which then cools to form a stable glass. Bycarefully controlling the vitrification process, the resulting vitrifiedglass may be made to exhibit great stability against chemical andenvironmental attack, with a high resistance to leaching of thehazardous components bound up within the glass. In this manner,vitrification may be utilized to convert waste materials into a highquality synthesis gas and a stable, environmentally benign, glass.

While systems utilizing plasma present significant advantages over priorart steam reforming systems, there still exists a need to minimize theenergy consumption and capital cost of these systems to increase theireconomic attractiveness. In particular, the energy required to effect aphase change to form the steam injected in these systems can increasethe costs of operating these systems significantly. Thus, there exists aneed for more efficient and improved methods of producing synthesis gasfrom organic and heterogeneous feedstocks.

SUMMARY OF THE INVENTION

Accordingly, the present invention is an improved method for processingorganic and heterogeneous feedstocks. It is an object of the presentinvention to provide a system that is capable of treating mixtures ofinorganic materials, biomass, and fossil-based organic materials andtheir derivatives, including waste derived from the production and useof such fossil-based organic materials, and to convert them into a cleanfuel gas and an environmentally stable glass. It is a further object ofthe present invention to provide a method and apparatus for processingorganic and/or heterogeneous feed stocks by providing a gasificationunit which converts all or a portion of the organic components of wasteto a hydrogen rich gas and ash, in communication with a joule heatedvitrification unit which converts inorganic materials and ash formed inthe gasification unit into glass, and a plasma which converts carbon andproducts of incomplete gasification formed in the gasification unit intoa hydrogen rich gas. The resultant glass may be used as a usefulproduct, such as a construction material, road aggregate or the like.Most importantly, the present invention does so in a manner thatovercomes the tradeoffs associated with traditional gasificationsystems, and provides these benefits in a compact system that minimizesboth the capital cost and the energy required to run the system.

The present invention accomplishes these and other objectives bycombining a gasification unit with a joule heated vitrification unithaving a plasma. Organic or heterogeneous mixtures of organic andinorganic feed stocks are first fed into the gasification unit where allor part of the organic portion of the feed stock are gasified. To assistin gasification, it is preferred that the materials be mixed with oxygenin the gasification unit. It is therefore preferred that thegasification unit have a one or more oxidant ports, for the introductionof an oxidant. Suitable oxidants include, but are not limited to, pureoxygen, defined as oxygen between 90 and 99% purity, air, carbondioxide, oxygen enriched air, steam, and combinations thereof.

Within the partial oxidation gasification system, all, or preferablyjust a part, of the organic portion of the feed stock is gasified. Theeffluent from this gasification process thus includes a gaseous portion,principally made up of carbon dioxide, hydrogen, and light hydrocarbongasses, together with a solid and liquid portion, which includesunreacted and partially reacted organic materials such as carbon char,together with the inorganic portion of the feed stock, which may alsoinclude ash from the gasification process.

The effluent is then fed directly from the gasification system into ajoule heated plasma reaction chamber to pyrolize and gasify theremaining solid and liquid organic materials, and to allow sufficientresidence time and mixing to form the ash and other remaining inorganicportions of the feed stock into stable, vitrified glass.

Typically, the feed stock utilized in the present invention is in theform of a solid, liquid, slurry, or mixture thereof. As practiced in thepresent invention, a feedstock is first introduced into a gasificationunit. Both updraft type and downdraft type gasification chambers aresuitable for practicing the present invention. The effluent is then fedto a joule heated vitrification unit wherein some or all of theremaining organic portion of the feed stock is pyrolized by exposing thefeed stock to a plasma. While it is preferred that the plasma be formedby plasma electrodes, a plasma torch, including but not limited to asteam torch, may also be used.

The plenum space within the joule heated vitrification unit may bemaintained as a reducing environment by eliminating or preventing theingress of any additional ambient air or oxygen and maintaining anoverpressure of an inert gas such as nitrogen. Alternatively, furtherpartial oxidation may occur in the plenum space of the joule heatedvitrification unit through the introduction of additional oxygen.Whether operated in a reducing environment or in a partial oxidationmode, steam or carbon dioxide may be added to the joule heatedvitrification unit as a further oxidant.

Typically, plasma gasification of the unreacted portion of the organicportion of the feedstock is accomplished in a two-step process. First,the material is gasified by the plasma in the joule heated vitrificationunit to its elements, primarily carbon, hydrogen and carbon monoxide,due to the intense heat from the plasma. Second, any carbon char formedreacts with steam, carbon dioxide or free oxygen to produce additionalhydrogen and carbon monoxide. By using bound oxygen in steam or carbondioxide as the oxidant rather than air, the joule heated vitrificationunit atmosphere may remain highly reducing.

If steam is used to gasify carbonaceous material, as in standardgasification systems, hydrogen and carbon monoxide are the primaryproducts exiting the process chamber. These gases are formed by thefollowing reactions:C_(x)H_(y(s)) +xH₂O_((g))→(y/2+x)H_(2(g)) +xCO_((g))  (1)C_(x)H_(y)O_(z(s)+() x−z)H₂O_((g))→(y/2+(x−z))H_(2(g)) +xCO_((g))  (2)C_((s))+H₂O_((g))→H_(2(g))+CO_((g))  (3)

In the present invention, if carbon dioxide (CO₂) is substituted forwater in reactions 1 through 3 either in part or completely to alsoproduce a carbon monoxide and hydrogen gas mixture according to thefollowing set of reactions:C_(x)H_(y(s)) +xCO_(2(g))→(y/2)H_(2(g))+(2x)CO_((g))  (4)C_(x)H_(y)O_(z(s))+(x−z)CO_(2(g))→(y/2)H_(2(g))+(2x−z)CO_((g))  (5)xC_((s)) +yCO_(2(g))→(x+y)CO_((g))  (6)

If excess steam is present, reactions 1 through 3 will proceed furtherto yield additional hydrogen and carbon dioxide. As will be apparent tothose having skill in the art, in addition to steam, the carbon dioxideused in the present invention may readily be supplemented with elementaloxygen introduced in a pure form or as air to partially oxidize theorganic feedstock.

The temperatures generated within the plasma in the joule heatedvitrification unit will typically range between 3,500° C. and 10,000° C.The temperatures in the surrounding plenum space are somewhat lower.While any plasma generating device, including but not limited to an arcplasma system and a plasma torch, is suitable for the present invention,arc plasma systems utilizing graphite electrodes are preferred. Inconjunction with the plasma, a joule heating system is utilized toprovide deep, even volumetric heating to the glass bath which forms fromthe inorganic portions of the feed stock in the joule heated plasmareaction chamber.

Any unreacted material leaving the joule heated vitrification unit maybe further processed in a thermal residence chamber. The thermalresidence chamber maintains the effluent of the joule heatedvitrification unit for a time and at a temperature suitable to completethe reactions necessary to convert the remaining carbonaceous materialsinto carbon monoxide.

The present invention further includes a feedback control device tocontrol the flow of oxygen, steam, carbon dioxide, air, and combinationsthereof through the oxidant injection port(s) to optimize the conversionof said organic components into a hydrogen rich gas. Sensors fitted tothe gasification unit, the joule heated vitrification unit, and thethermal residence chamber are used to measure the temperature, gascomposition, flow rates, and the like. As will be recognized by thosehaving ordinary skill in the art, such sensors, including but notlimited to, thermocouples, rotometers, flowmeters, oxygen sensors,hydrogen sensors, carbon monoxide sensors, and the like are wellunderstood and routinely used to monitor chemical and industrialprocesses. By monitoring the flow, temperature, and quality offeedstocks, oxidants, and gaseous effluent, the process may beautomatically controlled by adjusting the various parameters with afeedback control device.

For example, and not meant to be limiting, it is typical that completeoxidation in the gasification unit will generate a gaseous effluent withhigh levels of undesirable constituents, such as tars. The feedbackcontrol device would be configured to recognize that complete combustionwas occurring in the gasification unit by measuring a combination of theparameters used to operate the gasification unit. For example, and notmeant to be limiting, by measuring effluent gasses, the flow rates ofthe feedstock, and the flow rates of the oxidant the feedback controldevice could determine that complete combustion was occurring in thegasification unit. Having recognized an undesirable operation, thefeedback control device could then increase the feed rates for one orboth of the oxidant or the feedstock, thereby preventing completecombustion in the gasification unit.

For example, if the gasification unit is configured as a downdraftgassifier, the feedback control device could control a means fortransporting organic material down the axial length of the downdraftgasifier. In this manner, the flow rate of the feedstock through thegassifier could be increased or decreased. Suitable means fortransporting organic material down the axial length of the downdraftgasifier would include, but not be limited to, an auger, a rake, anagitating grate, one or more rotating drums, a piston, and combinationsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the inventionwill be more readily understood when taken in conjunction with thefollowing drawing, wherein:

FIG. 1 is a schematic illustration of the apparatus of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, FIG. 1 provides a schematic illustration of the apparatus ofthe present invention configured with a downdraft gasification unit andan electrode plasma source. While this particular configuration ispreferred, the present invention should in no way be limited to thisconfiguration, and it should be understood that this configuration wasselected merely for illustrative purposes.

As shown in the figure, organic materials are fed into a gasificationunit 1. Oxidants, including but not limited to, oxygen, steam, carbondioxide, air, oxygen enriched air, and combinations thereof, are fedinto oxidant injection port 2. The gasification unit 1 is then operatedas a normal, downdraft gassifier. A means for transporting organicmaterial down the axial length of the downdraft gasifier is shown in theschematic as box 3. Any mechanical means suitable for moving solidmaterial may be used, including, without limitation, an auger, a rake,an agitating grate, one or more rotating drums, a piston, andcombinations thereof.

Organic materials are preferably partially gasified in gasification unit1, resulting in a hydrogen rich gas, a partially oxidized organicmaterials, and ash, which are then transferred to joule heatedvitrification unit 4. A plasma 5 is created by plasma electrodes 6, andorganic materials from gasification unit 1, are immediately exposed toplasma 5 upon entering joule heated vitrification unit 4. Inorganicmaterials present in heterogeneous feedstocks are incorporated intoglass bath 7, which is generally maintained in a molten state by jouleheating electrodes 8. Any unreacted organic materials are finallyconverted into a hydrogen rich fuel gas in thermal residence chamber 9,by maintaining the materials for a time and at a temperature sufficientto complete the required reactions. A feedback control device 10monitors and controls variables such as the material flow rates,temperature, and gas quality, to insure complete processing of the wasteinto the glass and a hydrogen rich fuel gas.

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

The invention claimed is:
 1. A gasification/vitrification systemcomprising: a downdraft gasification unit configured to receive amixture including organic and inorganic materials, the gasification unitconfigured to generate an effluent from at least a portion of theorganic materials, wherein the effluent comprises at least a gas portionand a solid portion, wherein the solid portion comprises an incompletelygasified portion of the organic materials; and a vitrification unitconfigured to receive at least a portion of the inorganic materials andat least some of the gas portion and solid portion of the effluent fromthe downdraft gasification unit, and wherein the vitrification unitincludes: first electrodes configured to generate a plasma within thevitrification unit; and second electrodes configured to provide jouleheating to the vitrification unit; wherein the vitrification unit isconfigured to produce hydrogen-rich gas and glass from the inorganicmaterials and the incompletely gasified portion; and wherein thedowndraft gasification unit is positioned above the vitrification unitfor feeding the incompletely gasified portion directly from thedowndraft gasification unit into the vitrification unit.
 2. Thegasification/vitrification system of claim 1, further comprising a gratedisposed between the downdraft gasification unit and the vitrificationunit.
 3. The gasification/vitrification system of claim 2, wherein thegrate is an agitated gate.
 4. The gasification/vitrification system ofclaim 1, wherein the system further includes a transport mechanism formoving at least a portion of the mixture in an axial direction of thedowndraft gasification unit.
 5. The gasification/vitrification system ofclaim 4 further comprising a controller coupled to sensors incorporatedwithin the gasification unit and the vitrification unit, wherein thecontroller is configured to control the transport mechanism to vary arate of movement of the at least a portion of the mixture based, atleast in part, on operating parameters of the downdraft gasificationunit.
 6. The gasification/vitrification system of claim 1 furthercomprising one or more injection ports operable to provide an oxidantinto the downdraft gasification unit.
 7. The gasification/vitrificationsystem of claim 6 further comprising a controller operable to controlthe flow of the oxidant through said one or more injection ports based,at least in part, on operating parameters of the downdraft gasificationunit.
 8. The gasification/vitrification system of claim 7, wherein thecontroller is configured to control the flow of the oxidant to preventcomplete combustion in the downdraft gasification unit.
 9. Thegasification/vitrification system of claim 1 further comprising athermal residence chamber in communication with the vitrification unit.10. The gasification/vitrification system of claim 5, wherein thecontroller comprises a feedback control device configured to measure acarbon monoxide level in the gasification unit.
 11. Thegasitication/vitrification system of claim 5, wherein the sensorscomprise a carbon monoxide sensor.
 12. The gasification/vitrificationsystem of claim 11 further comprising a grate disposed between thedowndraft gasification unit and the vitrification unit, wherein afeedback control system is configured to control the grate to increaseor decrease the flow rate of organic material down the axial length ofthe gasification unit responsive to an output signal of the carbonmonoxide sensor so as to avoid complete combustion of unreacted organicmaterials including carbon char in the gasification unit.
 13. Thegasification/vitrification system of claim 1, wherein the effluentfurther comprises a liquid portion.